Takagi 110U Indoor (T-KJr2U-IN) Installation Manual And Owner's Manual page 30

APPLICATIONS
In order to purge air in water pipes within a closed-loop system, an air vent
and air separator should be installed in the system. Required circulation
flow rates are labeled next to each application diagram. These flow rate
WARNING requirements must be followed
*The recirculation pump is to be controlled by:
-Dual-set aquastat (recommended w/timer)
each activated unit in the system.